1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to a semiconductor device which includes redundant circuitry having fuses therein.
2. Background Art
In recent years, semiconductor devices have been miniaturized and increased in capacity and speed, resulting in an increase in the number of devices formed therein. This has increased the influence of the device defective portions on manufacturing yield. Therefore, to ensure adequate manufacturing yield, many semiconductor devices have a redundant circuit configuration in which a plurality of spare (or redundant) memory cells are formed among ordinary memory cells.
With this arrangement, if a memory cell is found to include a defective bit, the defective bit is replaced by a spare memory cell (provided beforehand) to salvage the semiconductor device. One widely used method for replacing a defective bit by a spare memory cell is known as laser trimming, which cuts off a wiring layer(s) (functioning as a fuse) by use of a laser beam.
If laser trimming is applied to a semiconductor device structure in which wiring such as signal wiring and power supply wiring and internal devices such as transistors are formed in the layers under the fuses, these internal devices may be damaged when a fuse is blown by the laser beam. Therefore, semiconductor devices are often configured such that the region right under fuses and regions therearound have no devices other than fuses formed therein.
Further, to prevent defects due to a fuse blow by laser trimming, a method for blowing a fuse is proposed which applies a plurality of different wavelength laser beams to fuse wiring having a multilayer structure (see, e.g., Japanese Laid-Open Patent Publication No. 2002-134616).
However, as described above, in the above structure that no devices other than fuses are formed in the regions right under fuses and regions therearound, which means that these regions are available only for formation of fuses. This may restrict miniaturization of semiconductor devices. To address this problem, a blocking layer may be provided right under the region in which fuses are formed in order to block the laser beam. Since this blocking layer does not allow the laser beam to pass, devices can be formed in the regions under the blocking layer.
Incidentally, in general, the fuses and wiring in the same layer are formed at once. This means that the fuses may be formed of the same material as the wiring (copper, etc.). However, copper, for instance, has low laser light absorptivity. A high energy laser beam is needed to cut off a fuse formed of a material having low laser light absorptivity (such as copper). When the above blocking layer (for blocking a laser beam) is irradiated with such a high energy laser beam, however, the layer may melt and hence the laser beam may reach layers under the blocking layer.
Further, with the miniaturization of semiconductor devices, the distances between the fuses have decreased. Therefore, when blowing a fuse by laser irradiation, it is necessary to prevent the adjacent fuses from being accidentally blown. Further, the opening formed as a result of blowing a fuse may reach the oxide film on an adjacent fuse or even the adjacent fuse itself, exposing them. In such a case, the exposed adjacent fuse (made of metal) may corrode, resulting in an electrical disconnection.